First, let's look at what we know for certain. Gliese 581 is a red dwarf star -- a star smaller and cooler than the sun. Gliese 581 is about 21 light-years away and was already known to have a Neptune-mass planet (Gliese 581b) orbiting the star every five and a third days. Astronomers knew this because the star is wobbling as the gravity of Gliese 581b pulls on it, and we can measure this wobble. But one planet didn't fully explain the observed wobbles. After more observations, the European astronomers figured out that the system must also have a planet at least five times the mass of the Earth orbiting the star every 13 days.

Really, that is all we know. Anything else is guesswork, though these guesses have some scientific basis and are not just idle speculation. If the Gliese 581c is made of rock, its diameter will be about 1.6 or times that of the Earth, and the pull of its gravity will be about 1.6 times stronger than that of the Earth, meaning that if you weigh 180 pounds on the Earth, you'd weigh 290 pounds on Gliese 581c. However, it could be made of ice, and therefore be larger, or it could have a lot of gas (like Uranus or Neptune), in which case it could be quite a bit larger. We don't know.

We also know that, at the distance that Gliese 581c is from its parent star, it lies in a "sweet spot" where temperatures are just about right for water to exist as a solid, liquid, and a gas, as it does here on the Earth. Scientists call this the "Habitable Zone," since life as we know it could live there. However, just because a planet is in the Habitable Zone around a star doesn't mean the planet could support life. Maybe it doesn't have any water. Maybe it has an atmosphere like Venus and is a thousand degrees on the surface. Maybe it doesn't have an atmosphere at all. We don't know.

Another issue with Gliese 581c is that it is so close to its parent star that the same side of the planet almost certainly faces the star, just like the same side of the moon always faces the Earth. This would make one side of the planet fairly hot, and the other side frigid cold (although an atmosphere could help mitigate this effect). It may be that all the water on the planet is locked in giant ice caps on the side of the planet away from the star. We don't know.

In short, Gliese 581c is a very interesting discovery, and the astronomers who found it have made a very important find. But the find illustrates how little we know about planets in other solar systems. It will likely take years before we learn much more about Gliese 581c, and it may be decades or longer before we can ever separate its light from the light of its parent star (separating the light would make it easier to study its atmosphere). But you can be certain that astronomers will continue to study this planet and to search for more and more Earth-like planets around other stars!

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The Professor Astronomy Blog is based on work supported by the National Science Foundation under grant no. AST-0602288. Any opinions, findings, and conclusions or recommendations expressed in this material are those of the author and do not necessarily reflect the views of the National Science Foundation (NSF).